This invention relates to a nuclear magnetic resonance inspection apparatus and its method.
More particularly, the present invention relates to a magnetic resonance imaging (hereinafter referred to as "MRI") apparatus for visualizing a density distribution of nuclei, a relaxation time distribution, etc, by measuring nuclear magnetic resonance (hereinafter referred to as "NMR") signals from hydrogen, phosphorus, etc, inside a living body, and specifically to a radio frequency (RF) coil for generating or receiving an RF magnetic field.
To begin with, the function of this RF coil and signal transmission means will be explained with reference to FIG. 1 which is a block diagram showing the construction of an MRI apparatus according to the prior art.
A coil 18 generates a static magnetic field having a predetermined field intensity in a predetermined direction (z-axis direction) inside a space by a current supplied thereto from a power supply 19. Three sets of gradient magnetic field coils 12, 13, 14 are driven by gradient field magnetic field coil drivers 15, 16, 17, respectively, and provide gradients in the z direction and x and y directions orthogonally crossing the z direction to the intensity of the static magnetic field. An RF coil 8 formed on a cylindrical bobbin is disposed in these coils. A movable bed is fitted to a support couch 22, and an object 20 to be examined is inserted into the RF coil 8 while lying on the bed 21. The output of an RF transmitter 6 is amplified by an amplifier 7 for transmission and is then applied to the RF coil, so that the resulting RF magnetic field is irradiated to the object 20. A controller 5 controls the driving timing of each gradient magnetic field coil and the generation timing of the RF magnetic field in accordance with a programmed sequence, and excites the nuclear spins of the object. RF signals resulting from nuclear magnetic resonance of the object are detected by the RF coil 8, and is guided to a signal processor 11 through a reception amplifier 9 and a quadrature phase detector 10. The signal processor executes processing of the reception data which are sampled, and converts them to an image. In this way, the function of the RF coil 8 is to excite the nuclear spins of the object and to detect the NMR signals emitted when the excited nuclear spins return to a steady state.
A signal transmission route at the time of the irradiation of the RF magnetic field extends from the generation of the irradiation signal by the RF transmitter 6 to the amplification of the signal by the transmission amplifier 7 to several kilo-Watts and the transmission of the signal to the RF coil. On the other hand, the signal transmission route at the time of the reception of the NMR signal extends from the detection of the signal emitted from the object 20 by the RF coil to the amplification of the detected signal of several micro-Volts by the reception amplifier 9 and to the transmission to the quadrature phase detector 10. Signal transmission between the members 6 and 7, between 7 and 8, between 8 and 9 and between 9 and 10 is carried out through coaxial cables. The difference between the irradiation operation and the detection operation lies in that whereas a thick cable for large power must be used for the irradiation cables, a thin cable can be used for detection.
Two kinds of transmission/reception systems are available for the RF coil. One is a single coil system which performs both irradiation and detection by one RF coil, and the other is a cross-coil system which separately performs irradiation and detection by two RF coils. The RF coil shown in FIG. 1 is the single coil system. The single coil system is ordinarily used for the whole body coil, and the coil is installed inside a magnet, while the cross-coil system is used for the head coil, the surface coil, and so forth.
However, the prior art example described above is not free from the following problems.
First, if a coaxial cable is connected to a detection RF coil or to an RF coil for both irradiation and detection, the coaxial cable itself plays the role of an antenna and an external noise is more likely to be caught i.e., experienced therein. As a result, a signal-to-noise ratio (S/N) of an image drops. When a part of the coaxial cable is grounded to an electromagnetic shield so as to reduce any external noise, the mode of electromagnetic coupling of the electromagnetic shield and the RF coil becomes different, and the characteristics of the RF coil change unavoidably.
On the other hand, the detection RF coil of the cross-coil system is disposed in close contact with the object, or in such a manner as to encompass the object, in order to receive the NMR signals emitted from the object. Therefore, the detection RF coil must be moved in accordance with an imaging portion. If the coaxial cable is connected to the RF coil in this case, not only movement becomes difficult but also the possibility occurs that the coaxial cable is clamped between movable portions. The RF coil must be exchanged in many cases in accordance with the imaging portions and in such a case, the coaxial cable must be removed and be connected once again. Accordingly, the possibility of erroneous connection or inferior connection develops.
A system for solving these problems is described in JP-A-1-223943 and JP-A-63-194648. However, these prior art references essentially include the connection between the coil and the cable, and cannot completely solve above-mentioned problems.